The below tests were of a single frame of Fujichrome Velvia transparency film.  There was a slight curl to the film towards the emulsion side.  This often happens after film processing.  To achieve the best edge to edge sharpness with any of the Nikon Scanners, it is my experience that the film needs to be as flat as possible. Or as shown below, that you find the best method of achieving flat film through a slide mount or film holder.  

For slightly curled films, the best solution that I have found is the QuickPoint slide mount.  It is made by the Loersch Corp., http://www.loersch.com . The mounts that I used are described on page 6 of the Loersch USA catalogue as "Press Fit"  24 mm x 36 mm aperture, 1.4 or 1.9 mm, Glassless.  I mainly use the 1.9 mm thickness mounts. They come in boxes of 100. A box of 100 is presently listed as $4.00 US.

Your own experimentation with achieving flat film may be necessary to achieve results acceptable for your intended outputs or personal standards.  If you scan perfectly flat film, then probably any type of slide mount or film holder will provide excellent edge to edge sharpness.  I do not find fault with the Nikon ED 4000 scanner regarding edge to edge sharpness but rather with the amount of curl that some films take on after processing or in storage or due to humidity. There may be other ways to achieve flat film but these results are what I've found to date.  I'd be happy to hear of any successful results that you achieve with your tests.   Email: %61%6c%65%63%40%70%79%74%6c%6f%77%61%6e%79%2e%63%6f%6d

Lately, I have been contacted by half a dozen people at least, wondering about the sharpness of their scanners.  I arranged with one gentleman to send me his slide to scan.  The main problem for sharpness was the slide mount that he was using.  It just did not hold the film flat and it was a little thicker than the 1.9 mm QuickPoint mounts that I use.  The other problem that he had, was that the image was not as sharp as it could have been. This could have been due to many reasons which were not determined. The lens could have been an inferior brand or version, too wide an f stop was used, no tripod was used, camera shake occurred, or other reasons.  Make sure that you have a very good and focusable loupe to examine your original slides. I do my rough edit with a Pentax 4x focusable loupe and I do my critical sharpness edits with a Peak 10x focusable loupe.  The Peak 8x or Nikon non focusable loupes or similar magnification, is the minimum that I would use for determining the sharpness of the original film.   Also realize that a scan is a second generation and sharpness deteriorates with every generation.  Perceived sharpness can be brought back by using the Nikon Scan USM mode, or in Photoshop's Unsharp Mask, which is what I use.